What is the jet stream?

The jet stream is a fast flowing, river of air found in the atmosphere at around 12 km above the surface of the Earth just under the tropopause.

They form at the boundaries of adjacent air masses with significant differences in temperature, such as of the polar region and the warmer air to the south.

The Earth's rotation effects the direction of the jet steam causing it to flow from west to east, propagating in a serpentine or wave-like manner at lower speeds than that of the actual wind within the flow.